Rioplatense Spanish or River Plate Spanish (Spanish: español rioplatense, although locally known as castellano rioplatense) is a dialectal variant (or simply, a "dialect") of the Spanish language spoken mainly in the areas in and around the Río de la Plata basin (or River Plate region) of Argentina and Uruguay, and also in Rio Grande do Sul, although features of the dialect are shared with the varieties of Spanish spoken in Eastern Bolivia and Chile. The usual word employed to name the Spanish language in this region is castellano (English: Castilian) and seldom español (English: Spanish) (see: Names given to the Spanish language). Note that while this article refers to Rioplatense Spanish as a single dialect, there are distinguishable differences among the varieties spoken in Argentina and in Uruguay, as described below.

Rioplatense is mainly based in the cities of Buenos Aires, La Plata, Mar del Plata, Santa Fe and Rosario in Argentina, Montevideo in Uruguay, and far south of Rio Grande do Sul in Brazil, the seven most populated cities in the dialectal area, along with their respective suburbs and the areas in between. This regional form of Spanish is also found in other areas, not geographically close but culturally influenced by those population centers (e.g., in parts of Paraguay and in all of Patagonia). Rioplatense is the standard in audiovisual media in Argentina and Uruguay. To the north, and northeast exists the hybrid Riverense Portuñol.